Plug plants in horticulture are small-sized seedlings raised in growth media in small, individual containment units and ready to be transplanted into larger containers, a nursery field, a landscape or a garden. Plug plants are typically raised in controlled conditions during their important formative period (the first 4-6 weeks) to help ensure plant health and for the plants to reach their maximum potential during the harvest/blooming period. Propagation trays having a plurality of containment units are generally used to raise a plurality of plug plants for convenience of storage and transport. Plug plants are used for commercially raising vegetables, bedding plants, trees and the like.
Improper propagation tray design can significantly alter root formation of plants, especially of woody perennials, leading to substantial deformation, which is challenging and expensive to correct during later stages of production often resulting in malformed root balls. Root deformation to the structural roots of trees makes them less robust after transplanting, increasing tree mortality because roots are not placed advantageously for survival and establishment. Additionally, kinked, deformed and/or deep roots make trees susceptible to weather events posing serious safety risks, which is a major concern for urban forestry and park managers in cities around the world. Root defects generally occur when the design of the container or propagation tray inhibits the lateral roots from extending horizontally, forcing the roots to either circle within the container or grow vertically down to the bottom. Unfortunately, root deformation is common in plug-grown seedlings and if not corrected through mechanical remediation before out-planting, can contribute to long-term tree growth problems in the landscape.
Currently there are three main ways to manage root deformation: mechanical remediation (manual pruning), chemical pruning, and air-root pruning.
Mechanical remediation of circling roots at transplanting has become a recommended practice; however, root pruning to correct root malformation can cause transplant shock during field establishment. Root shaving (i.e. shaving off outer few inches of a root ball to remove circling roots) has been introduced as a method of correcting root malformations; however, at the propagation and liner stages it is extremely time consuming and labor intensive, and is therefore cost prohibitive. Additionally, mechanical pruning can result in human error (missing deformed roots in the interior of the ball) and removing mostly young active roots that are useful in helping trees become established.
Chemical pruning typically involves using containers coated with cupric carbonate, copper hydroxide or cupric sulfide to manage root development (copper salts in a latex carrier). This method has had widespread use in forestry (e.g. pretreated Styrofoam™) since the 1960's. As roots develop in the container and then reach the container wall, the chemical coating kills the root tips. Originally the product was delivered in the form of a user-applied coating, which producers found effective but inconvenient and labor-intensive. Now, a pretreated product line is available. Not all species respond the same to the treated pots and a proper application rate of the treatment is important to avoid excess root control and nutrient imbalance (i.e. copper-induced chlorosis). Nutrient uptake imbalances are particularly prevalent in containers with a high ratio of container surface area to substrate volume, such as propagation trays. Plants can also become stunted if they are left too long in the containers before shifting. Additionally, some growers are reluctant to use the product because of perceived environmental effects. Also, longevity of the coating is questionable and the coatings are expensive. Copper-treated trays do not have drainage holes so plugs can become waterlogged and aeration is reduced.
Air-root pruning involves desiccation of the root tips on exposure to air resulting in a loss of root apical dominance and the development of more fine roots. Many specialized container types have been designed to use air-root pruning involving specialized container shapes, bottomless containers, woven or non-woven fabrics, mechanical deflection or chemicals to control root growth. Air-root pruning technology has also been combined with mechanical deflection by including vertical ribs on the interior or exterior of (interior grooves) container surfaces.
In one example of an air-root pruning container, root growth and development is managed using a porous walled container, which stops root growth at the wall-substrate interface. As the roots grow towards the container walls the openings expose the substrate and consequently the root tips to air and the tips are desiccated (air pruning). As a result of the desiccated tips, branching occurs behind the root tip causing more fine roots to develop in the inner part of the root ball which allows for a more even root distribution and root systems to develop. However, at the larger sizes of nursery stock for which these porous-walled containers are created, the containers do not correct the malformed roots that develop in the propagation stage and manifest as permanent defects to structural roots as a result of the imprinting of the propagation plug cell on the root structure during early development.
Various other air-root pruning trays that exist on the market today have features that result in malformed roots. Even trays that are designed to air-root prune plugs to avoid circling roots still have structures that allow for root “trapping” and deflections mainly because plastic structures featured on the trays come into contact with the growing medium and roots become trapped by the vertical/horizontal ribbing or grooves and are redirected. Propagation trays on the market today having a typical range of soil volume for propagation (105 cm3 to 547.52 cm3) and have structures making significant contact with the plug will result in root malformation. Even trays having strategically placed holes with the objective of creating non-circling roots will still cause descending roots and therefore still result in malformed root architecture.
There remains a need for improved propagation trays to reduce the incidence of deformed roots during propagation.